Various transducer circuits such as that disclosed in U.S. Pat. No. 3,597,750, issued on Aug. 3, 1971, and assigned to the same assignee, are used for detecting the position of movable members or other such apparatus. In this patent a plurality of light emitting diodes are positioned adjacent a grid which includes spaced opaque lines mounted on a movable member. Photo diodes (light detectors) are positioned on the opposite sides of the grid such that as an opaque line passes between the LED and the photo diodes, there is detected a light change for enabling the sensing of the position corresponding to that opaque line.
As evidenced in this patent, problems are encountered because the light output generated by LEDS and other such light sources changes with usage. Also, since the characteristics of light transducers are different from device to device, it is impossible to maintain an equal output signal from each transducer by using a fixed control signal. That means certain adjustments of the control signal have to be made for each transducer in order to achieve the desired output signal which adjustments are very tedious and expensive. However to maintain the system at a high operating efficiency it is desirable to sustain the light output at a desired level. Naturally the transducers will not indicate whether such gradual changes in the light are due to the positioning of the grid or to a deterioration in the light source. In order to assure that the changes in light value detected are always due to changes in position of the movable member only, the light emitted from the light emitting diode must be maintained substantially constant. To maintain the light emitted constant the transducer gain must be detected in some manner, however there is no available reference with which to compare this gain, which is the ratio of the transducer output value to the grid spacing (position) being measured. But since the output signal is cyclic with movement of the grid, use can be made of the momentarily measurable peak value to set the transducer gain automatically.
It is the purpose of this invention to sense the gain of such a light transducer and provide a circuit for readjustment thereof to maintain the output signal at a desired level.